1. Field of Invention
The invention relates to a switch driving circuit. More particularly, the invention relates to a switch driving circuit for driving a full-controlled power switch.
2. Description of Related Art
The insulated gate bipolar transistor (IGBT) is one of the full-controlled power switch elements. The input stage and the output stage of the insulated gate bipolar transistor are integrated into one device respectively through the metal-oxide-semiconductor field-effect transistor (MOSFET) and the PNP bipolar junction transistor (BJT), which has both the features of low driving power, rapid switch speed (control and response) and high input impedance of the MOSFET device and the features of reduced saturation voltage and high capacity of the bipolar device. Currently, the insulated gate bipolar transistor has become the mainstream power device for the high power converter. As the link between the power circuit and the control circuit, the main function of the driving circuit is converting the control signal into the required power driving signal to ensure that the insulated gate bipolar transistor is turned on completely or turned off reliably. Accordingly, the driving circuit plays an important role in transferring the control signal precisely and ensuring the good performance of the full-controlled power switch element.
In the application of the high power converter and the medium power converter, along with the turning on or off of the full-controlled power switch element, a high variation gradient of voltage/current is generated and thus the corresponding noise interference is caused. In such an environment, the driving circuit is required to have a good anti-interference performance so as to ensure that the control signal is transferred precisely and thus the stable and reliable operation of the full-controlled power switch element is realized. At the same time, it is required that the power side and the control side of the driving circuit are well isolated, avoiding that the control circuit is damaged due to serious failure of the main circuit. Accordingly, the design difficulties of the driving circuit are the precise and rapid transmission of the control signal, strong anti-interference performance and good function of isolation between the main circuit and the control circuit.
Currently, in the industry many mature drivers can drive the full-controlled power switch element reliably. When the driving power is low and it is unnecessary to isolate the control signal from the power device, the design of the driving circuit is relatively simple. In the application environment of the high power, when it is necessary to isolate the control signal from the main power device, the common driving methods are as follows: optical coupler isolation driving, optical fiber isolation driving and pulse transformer isolation driving.
The optical coupler driving method has disadvantages as follows: the secondary side of the optical coupler is limited by the power supply source (the power supply for a high-speed optical coupler is generally 5 V), the time delay of the signal transmission is long, the isolation voltage of the primary side and secondary side is not high and the stability is low.
Currently, the optical fiber driving method is widely applied in the high voltage environment. However, it also has the disadvantages as follows: the power supply source of the receiver of the optical fiber is limited (generally 5 V), the stability of the optical fiber is lower than that of a magnetic core element as the plug port of the optical fiber is easy to deposit dusts, and the large number of the optical fibers may cause complicated system wiring and the increased manufacture cost.
Another method uses the pulse transformer isolation driving to drive the full-controlled power switch element. The method is mainly divided into two types: active and passive. The passive method is relatively simple which uses the pulse transformer to drive the full-controlled power switch element directly without the secondary-side power source. In the passive method, the pulse transformer should transfer the driving signal and the power at the same time so that the transformer shall have a volume large enough. As a result, the parasitic capacitance of the primary side and the secondary side of the transformer is increased correspondingly and the common-mode noise interference is serious, which are issues should be avoided in the application of the high power.
In the active method, the pulse transformer only provides the function of transferring the driving control signal and the secondary side provides the corresponding power source. Compared with the passive method, in the active method the magnetic core volume can be reduced. However, if the frequency of the control signal is reduced, the time of high voltage level of the driving control signal may last longer. In order to prevent the saturation of the magnetic core, it is necessary to increase the volume of the magnetic core correspondingly. Similarly, the parasitic capacitance of the primary side and secondary side is increased correspondingly. It can be seen that the volume of the pulse transformer is limited by the operation frequency in the active method.
However, whatever from the view of the active method or the passive method, the pulse transformer causes the issue of parasitic capacitance. In the high power environment, such a parasitic interference noise interferes with the driving control circuit greatly. More serious it may cause the driving circuit to generate an error signal and then the full-controlled power switch element is triggered to turn on or off, which damages the reliability of the system greatly.